1. Field of the Invention
The present invention relates to a valve timing control system for controlling timing of opening and closing an intake valve and an exhaust valve of an internal combustion engine, in accordance with an operation of the internal combustion engine. Above all, the present invention relates to the valve timing control system provided with a mechanism for detecting a rotational position of a cam shaft and the like.
2. Description of the Related Art
Japanese Patent Unexamined Publication No. Heisei 10 (1998)-252420 discloses a valve timing control system for varying timing for opening and closing an intake valve and an exhaust valve, by adjusting an angle for fitting a drive force transmitter to a cam shaft; where the drive force transmitter (such as a timing pulley and a chain sprocket) is rotatable synchronously with a crank shaft of an internal combustion engine, and the cam shaft has an external periphery formed with a drive cam.
As is seen in FIG. 14, the valve timing control system according to Japanese Patent Unexamined Publication No. Heisei 10 (1998)-252420 has a target plate 2 at a front end (left end in FIG. 14) of a vane rotor 1. Target plate 2 has a plate body 5 and a bottomed cylindrical wall 6. Plate body 5 is toroidal, and is formed with a plurality of protrusions 5A extending radially outward. Bottomed cylindrical wall 6 extends on an internal periphery of plate body 5, and has a bottom section which is integrally coupled to vane rotor 1 with a cam bolt 7. Moreover, bottomed cylindrical wall 6 has a cylindrical section protruding from a housing 8. Plate body 5 at a head end (left end in FIG. 14) of the cylindrical section of bottomed cylindrical wall 6 is disposed on a front side of housing 8.
Each of protrusions 5A of target plate 2 changes a detection wave form which is detected by sensor 3 when running across a front surface of sensor 3. One of protrusions 5A is wider than the other protrusions 5A for sensor 3 to make a distinction. In other words, with the valve timing control system in FIG. 14, wider protrusion 5A can be distinguished from the other protrusions 5A referring to a difference in the detection wave form. The other protrusions 5A can be distinguished by counting the number of detection waves after wider protrusion 5A has been detected.
In the valve timing control system in FIG. 14, however, the internal periphery of plate body 5 of target plate 2 is formed with bottomed cylindrical wall 6 that is connected to vane rotor 1 with cam bolt 7. Formation of bottomed cylindrical wall 6 makes it difficult to produce target plate 2. Obtaining good products with precise (and/or accurate) dimension requires increase in production cost.
For reducing production cost, target plate 2 is ordinarily formed through a press molding. Target plate 2 in FIG. 14 requires a drawing during the press molding for forming bottomed cylindrical wall 6 on the internal periphery of plate body 5. However, obtaining good dimensional precision (and/or accuracy) of bottomed cylindrical wall 6 in the axial direction is of difficulty only through the drawing, and thereby requires another machining of bottomed cylindrical wall 6 in the latter production step.
Target plate 2 of the valve timing control system in FIG. 14 (or ones similar to target plate 2) is preferably light in weight so as to prevent failures such as rotational shift (of target plate 2) attributable to an inertial force.
Therefore, for reduction in weight, target plate 2 of the valve timing control system in FIG. 14 has the toroid (in the vicinity of the center) as small as possible, leaving a long extension of radial protrusions 5A. However, one of protrusions 5A considerably wider (for distinction) than the other protrusions 5A restricts the reduction of target plate 2 in weight.